A New Dinucleating N , O Donor Ligand ( H 2 BPClNOL ) and the Structural and Magnetic Properties of two Diiron Complexes with the di-μ-Alkoxo Motif

Dois complexos binucleares de Fe III , obtidos a partir de um novo ligante polidentado (H2BPClNOL = N-(2-hidroxibenzil)-N-(2-piridilmetil)[(3-cloro)(2-hidroxi)]propilamina e contendo a unidade estrutural Fe III (μ-alcoxo)2FeIII , foram caracterizados estruturalmente através de estudos cristalográficos e magneto-químicos. Os complexos [Fe 2(BPClNOL)2(OAc)]ClO4 (1) e [Fe2(BPClNOL)2(H2O)2](ClO4)2.4H2O (2) diferem entre si pela presença de um grupo acetato em ponte entre os sítios de Fe III no complexo (1). O complexo contendo a ponte acetato apresenta constante de acoplamento antiferromagnético aproximadamente 20 % menor quando comparado ao complexo sem ponte acetato.


Introduction
In the last decades, the search of magnetic information on dinuclear iron complexes has received special attention. 1tudies of iron synthetic complexes have shown that the oxo bridge is responsible for the strong antiferromagnetic coupling (usually -50 > J > -200 cm -1 ) [2][3][4][5] and that complexes with alkoxo, phenoxo, or hydroxo bridges are weakly coupled (usually 0 > J > -30 cm -1 ) 4 .
The exchange coupling between the high-spin Fe III ions in dinuclear iron model complexes has been a powerful tool to infer about bridging groups that are present in iron enzymes 2,[6][7][8] .In this way, it was suggested that in the deoxy form, hemerythrin has a hydroxo bridge (J ≅ -13 cm -1 ) 6,9 while in the met form, an oxo bridge (J ≅ -134 cm -1 ) 9 mediates the strong antiferromagnetic coupling between the Fe III centers.In addition, on the basis of magnetic susceptibility studies of the oxidized form of purple acid phosphatases, a dinuclear Fe 2 III center bridged by a carboxylate and two hydroxo groups (J ≅ -15 cm -1 ) has been proposed 10 .
Accordingly to Gorun and Lippard 4 , the magnitude of exchange interaction in dinuclear iron (III)-complexes is strongly dependent on the shortest exchange pathway between the two metal centers.They have shown that the magnitude of the exchange coupling constant J is tied up with the structural parameter P through the following correlation: -J (cm -1 ) = A*exp (B*P [Å]), with A = 8.763*10 11 and B = -12.663.The parameter P corresponds to half the length of the shortest bridge in the complex.This relationship has been applied with some success for iron (III) oxo-bridged complexes 4 .Moreover, in a recent study, based on a semiempirical angular overlap model, Weihe and Güdel 5 have shown that the bridging angle Fe-O-Fe also influences the magnitude of the exchange coupling.They concluded that J and the Fe-O-Fe angle are correlated with a total spread, which is about half of the Fe-O dependence for a wide range of µ-oxo diiron complexes.However, no such correlations have been described for di-µ-alkoxo, -phenoxo and -hydroxo complexes.In an attempt to accumulate more data for further discussion of magneto-structural correlations for this type of complexes, we present here the synthesis of a new polidentade ligand (H 2 BPClNOL), which was skilfully used to synthesize complexes that allowed to study the influence of the Fe-O-Fe angle on the exchange coupling.

Results and Discussion
The dinuclear complex cation [Fe 2 (BPClNOL) 2 (OAc)] + consists of two six-coordinated iron centers, which are bridged by two alkoxo oxygen atoms (O 3 and O 4 ) from the ligand H 2 BPClNOL and one acetate group.The two iron atoms exhibit a distorted octahedral structure with an N 2 O 4 donor set where the nitrogen atoms from the tertiary amine and from the pyridine groups and the oxygen from the phenolate are in a facial arrangement.The bridging oxygen atoms from the alkoxo groups are bond asymmetrically to the two iron centers The iron atoms in the centrosymmetric, binuclear structure of [Fe 2 (BPClNOL) 2 (H 2 O) 2 ] 2+ are bridged by two alkoxo oxygen atoms of the two H 2 BPClNOL ligands.Each of these ligands also binds one Fe atom through two nitrogen atoms (the tertiary N atom and one from the pyridyl group) and one oxygen from the phenolate in a meridional fashion, differently from complex 1, in which the same group of atoms of H 2 BPClNOL are coordinated facially.Two water molecules in anti-configuration complete the distorted octahedral coordination around the Fe atoms.In complex 1 the Fe-O alkoxo bond distances (av.2.008 Å) are very similar to those observed in complex 2 (av.2.009 Å) whereas the Fe•••Fe distance of 3.122 Å in 2 is somewhat longer than that detected in 1 (3.041Å).Consequently the Fe-O alkoxo -Fe angles in 2 are larger (av.101.95 o ) compared to those in 1 (98.8 o ).This feature represents the major difference between the bridging units in these complexes.Variable temperature magnetic studies of powder samples of 1 and 2 between 4.5 and 300 K show that the two Fe III centers are weakly antiferromagnetically coupled in both complexes (Figure 2).Very good agreement between theory and experimental data was obtained by using the following parameters: g = 2.0, J = -3.9(2) cm -1 , % imp = 0.0 and TIP = 400 x 10 6 cm 3 /mol for 1 and g = 2.0, J = -4.8(2) cm -1 , % imp = 0.07 and TIP = 400 x 10 6 cm 3 /mol for 2. These values of J lie in the range but at the lower end, of di-µ-alkoxo bridged Fe 2 III complexes 14- 16 .Moreover, a comparison between 1 and 2 reveals a small but significant increase (~ 20 %) in the J value with larger Fe-O alkoxo -Fe bridging angle.Since the shortest superexchange pathway [Fe-O alkoxo = 2.008 Å in the Fe 2 -(µ-alkoxo) 2 unit] and the coordination environment around the Fe III centers are very similar in 1 and 2, the increase in the Fe-O alkoxo -Fe bridging angle should be the main factor responsible for the increase of the coupling constant in 2. A direct interaction between the Fe III centers can be excluded for this type of complexes, due to the large metal-metal distance (over 3 Å). 17n the other hand, all attempts to correlate our data with Güdel's model 5 were unsuccessful.
Fe III (µ-alkoxo) 2 Fe III unit fixed.The presence of an additional acetate bridge in 1 has been shown to influence exclusively the Fe-O alkoxo -Fe angle and consequently the magnitude of the exchange coupling between the Fe III centers.The terminally ligated phenolate and the (µ-alkoxo) 2 (µcarboxylate) bridging unit in 1 also provide an interesting model for the oxidized form of purple acid phosphatases, which contains a dinuclear Fe 2 III unit, probably bridged by a carboxylate and two hydroxo groups 10 .The synthesis and structural characterization of further dinuclear Fe III complexes with H 2 BPClNOL and their magnetic properties are under way and will be the subject of future reports.In summary, we have synthesized and characterized two Fe 2 III complexes of the new polydentate ligand H 2 BPClNOL.This ligand is able to keep the dinuclear [Fe1-O 3 = 2.071(3), Fe 2 -O 3 = 1.959(3)Å and Fe1-O 4 = 1.925(3),Fe 2 -O 4 = 2.079(3) Å ].The Fe•••Fe distance is 3.041(1) Å and lies in the range of Fe•••Fe distances observed in other di-µ-alkoxo-Fe 2 III and di-µhidroxo-Fe 2 III complexes 4,7 .

Figure 2 .
Figure 2. Magnetic susceptibility and effective magnetic moment as a function of the temperature for complex 1 (a) and 2 (b).